1. Field of the Invention
The present invention relates to a pneumatic tire, and more particularly to a pneumatic radial tire for passenger cars having excellent riding comfort and stick slip resistance and capable of good run flat running even when the internal pressure is lowered.
2. Description of the Related Art
In the case of a relatively small radial tire such as a radial tire for passenger cars, an organic fiber such as a polyester, rayon and the like are generally used for a carcass ply material, and steel cord is used for the belt portion. Since suitable riding comfort is required in a tire for passenger cars, the steel cord used for the carcass ply material of tires for trucks and buses is not frequently used as a reinforcing material of the carcass ply of tires for passenger cars.
However, in recent years, it has been suggested that the steel cord be applied to the carcass ply of a radial tire of a relatively compact size.
For example, in Japanese Patent Application Laid-Open No. 62-137,202, a steel cord ply having a single layer construction with a filament diameter of 0.15 to 0.25 mm is described. However, in this example, improvement in riding comfort is still left unsolved, and since a stick slip phenomenon (a phenomenon in which the grip force of a tire is suddenly lowered when the angle of the steering wheel reaches a certain limit when suddenly turning the steering wheel while running, causing a side slip) is observed although so-called "(steering)stability" while (during) normal operation is improved, there still remains major problems in practical use.
Further, in Japanese Patent Application Laid-Open No. 63-265,704, there is described a steel ply cord in which flexural rigidity is lowered by setting the number of cord strands to between two to four and setting the diameter of the strands to between 0.15 to 0.21 mm. However, tensile stress at a specific extension (hereinafter, often referred to as a modulus) is high, compression fatigue problems have not been improved, and stick slip resistance performance is slightly improved but is far behind the level achieved by organic fiber.
Still further, in Japanese Patent Application Laid-Open No. 4-257,384, there is suggested a cord structure which guarantees the open characteristics of a 1.times.3 structure, however, even in this case, a riding comfort comparable to the level achieved by organic fiber is not reliably ensured, and the stick slip resistance performance is far behind the level of organic fiber.
Furthermore, in WO 97/14,566, it is suggested that a steel cord whose strength is improved be used for the ply cord. However, in this case, there is no consideration of flexural rigidity or compression rigidity of the cord although the weight of the cord is slightly reduced due to the use of a high strength steel material, therefore the stick slip performance mentioned above is likely to still remain a great problem. In addition, cord fatigue remains a major problem in view of guaranteeing safety.
In addition, in Japanese Patent Application Laid-Open No. 59-221,708, there is suggested a so-called strand ply cord construction using strands each having a strand diameter of 0.08 to 0.16 mm, and in Japanese Patent Application Laid-Open No. 1-30,803, there is suggested a 1+(1.times.n) construction, and in Japanese Patent Application Laid-Open No. 58-221,703, there is suggested a 3.times.3 strand construction and the like. However, the improvement achieved by these constructions is still far from overcoming the above-described disadvantages of the steel ply cord.
When using a steel cord for the carcass ply, the grip force of the tire is reduced for the following reasons. In general, if an organic cord is used as the carcass ply cord, when a great torsional deformation is generated in the tire, the belt layer undergoes a so-called bias deformation and absorbs the deformation by changing the angles of the reinforcing cord of a belt layer (hereinafter, the reinforcing cord will frequently be referred to as a belt cord). However, when the carcass ply cord is made of steel, since all three lines of a triangle formed by the two belt cords and a carcass ply cord are the steel cord which undergoes almost no compression, the bias deformation of the belt is restricted. That is, the torsional deformation of the tire can no longer be fully absorbed by the deformation of the belt portion, causing the tire tread surface to unavoidably float up from the road surface. As a result, the tire tread surface loses its grip of the road surface.
Accordingly, when the steering wheel is turned to more than a certain steering angle, the tire cannot withstand the deformation caused by such a wide steering angle, and the ply cord suddenly buckles and deforms, so that the type grip of the road surface is lost, causing a so-called stick slip phenomenon. This gives a great risk to the safety of the car and causes a lost of steering wheel stability when the steering wheel is suddenly turned in an emergency.
On the other hand, there is a disadvantage in the organic fiber (a polyester, a rayon, a nylon and the like mentioned above) having a low modulus (that is, a low tensile stress at a specific extension), which is generally used as the reinforcing material (a ply cord) of a carcass ply for a compact size tire having a relatively low internal pressure. Such types having a relatively low internal pressure are generally used for passenger cars or light trucks or the like. In the case of polyester, since the modulus is lowered under the high temperature when the tire is vulcanized, a so-called post cure inflation (hereinafter, referred to as a PCI) is required after the tire vulcanization process, therefore the productivity of the tire is deteriorated. Further, there are some concerns in the bonding between the cord and the matrix rubber composition, and decrease in the modulus of the cord due to the heat generated at high speed, so that the steering stability or durability of the tire may be deteriorated.
In the case of rayon, it is considered that the PCI is not required since the modulus is not lowered as much at the high temperature. However, since pulp is used for the raw material, there is a demand to replace the raw material with something else. Further, since sulfuric acid is used in the manufacturing process thereof, there might also be the problem of environmental pollution. Finally, as for nylon, since nylon is far behind in terms of the modulus compared to polyester and rayon, there is the disadvantage that the control stability is poor although it gives relatively good riding comfort. In addition, the PCI is indispensable in case of nylon.
Further, organic fibers of this kind require a bonding treatment (hereinafter, referred to as a dip treatment) for obtaining adhesion to the matrix rubber composition.
Still further, organic fiber causes a problem when it is used for a run flat tire developed in recent years and structured such that safe running can be ensured even when the tire is punctured.
When run flat tire is used, the user is able to safely move the vehicle to a place where the tire can be replaced even when the tire is punctured, and from the view point of the vehicle producer, since it is not necessary to mount a conventional spare tire on the vehicle, merits such as a wider space in the vehicle, reduced vehicle weight and the like can be obtained.
However, in the case of the run flat tire, since the tire runs in a state in which the internal pressure of the tire is lower, it has to experience a considerable amount of a distortion and deformation. Therefore, a high level of heat resistance is required for the tire such that it can overcome severe problems like melting of the rubber or even the cord, due to heat generated by the distortion and deformation of the tire.
In order to secure such heat resistance at a temperature equal to or more than 200.degree. C., not only the heat generation of the rubber needs to be restricted but also the heat resistance of the cord and that of the adhesion between matrix rubber composition and the cord must be improved. However, in the organic fiber generally used for the tire cord, there has been the disadvantage that the heat resistance of the cord itself or that of the adhesion between the matrix rubber composition and the cord is insufficient. When a steel cord is used in place of organic fiber, this particular disadvantage is improved, however, the other disadvantages inherent to the steel cord mentioned above are then caused.
In summary, since the steel cord has a higher tensile modulus and a higher compression modulus and thus is harder than normal organic fiber, a lot of advantages such as an improvement of strength, an improvement in heat resistance, and the like are achieved when the steel cord is used for the carcass ply cord of a radial tire having relatively low internal pressure and having a relatively compact size such as the tire for passenger cars and the like. However, on the other hand, the reduction in riding comfort, the increase in the tire weight, and the stick slip phenomenon caused by the belt rigidity being too high are generated. These give a great concern for the safety of the vehicle and could be a cause for losing the steering stability if the steering wheel is suddenly turned in an emergency.
On the other hand, when organic fiber is used instead of steel cord for the carcass ply cord material of the compact tire, there have been disadvantages in view of the productivity of the tire and the like.